Keep a leather dog collar in a dry, warm place. Put it into a special bag of natural fabric (cotton, linen). It will safe a leather dog collar (or other dog gear) from dust and excessive wet. The fabric should be ventilated. That's why natural cloth of bag is very important.
Do not wear leather dog gear on your dog in heavy rain. If a leather dog muzzle, harness or collar is got wet through, wipe it with a dry cloth and hang it to get dry in room temperature without use of heaters.
For colored leather dog gear straight sunlight is harmful. It may change color.
If you wish leather dog gear to keep shiny, flexible and soft, use a special leather care conditioner.